In general, a microphone refers to a device converting a sound such as a voice therearound into an electrical signal and processing the electrical signal to a signal enabling a human being or a machine finally to recognize the sound.
As microphones converting an audio signal into an electrical signal, capacitive and piezoelectric type microphones have been developed.
The capacitive type microphone includes a micro-electrochemical system (MEMS) in which a fixed membrane and a vibration membrane are spaced apart from each other. In the MEMS, when a sound pressure is applied to the vibration membrane, a space between the fixed membrane and the vibration membrane is changed to cause a capacitive value to be changed to generate an electrical signal, and the sound pressure is measured with the electrical signal.
Compared with an existing electrets condenser microphone (ECM), the capacitive type MEMS microphone is advantageous in that performance thereof is less changed over a change in an external environment such as a temperature or humidity and performance variations of products are small due to a semiconductor batch process.
Further, most capacitive type MEMS microphones have stable frequency response characteristics and excellent sensitivity. In the capacitive type microphone, a change in capacitance between a vibration membrane and a fixed membrane is measured and output as a voltage signal, and it is expressed as sensitivity, one of major performance indices.
In order to enhance sensitivity, it is designed to lower residual stress of a vibration membrane, and to this end, a free-floating membrane structure has been researched.
FIG. 1 is a view illustrating a free-floating membrane structure of a commercialized MEMS microphone according to a related art.
Referring to FIG. 1, a general free-floating membrane structure includes a vibration membrane 3, in a state of not being fixed, between a substrate 1 and a fixed membrane 2, deviating from a concept of an existing clamped capacitive type vibration membrane.
With this structure, since the vibration membrane 3 is not restrained between the substrate 1 and the fixed membrane 2 after all the processes are finished, a vibration membrane residual stress, one of the most important factors determining sensitivity of the MEMS microphone, may be removed.
In the vibration membrane structure without a residual stress, the vibration membrane 3 is fixed by electrostatic force based on an applied voltage (i.e., a driving voltage) by applying a rigid support post 4 to the free-floating membrane 3 and the fixed membrane 2.
In detail, when a driving voltage (bias) is applied to the microphone, the vibration membrane 3 is attracted toward the fixed membrane 2 due to an electrostatic force and attached and fixed to the support post 4 to vibrate by a sound pressure. When the driving voltage is not applied, the vibration membrane 3 is separated from the support post 4 and lowered to the substrate 1 because the electrostatic force is released.
However, when the related art microphone is not driven, the vibration membrane 3 is not fixed and free between the substrate 1 and the fixed membrane 2, and thus, the vibration membrane 3 may be damaged due to an impact.
Stress caused as the vibration membrane 3 is repeatedly brought into contact with or separated from the fixed membrane 2, and the support post 4 according to driving/non-driving of the microphone may degrade durability.
In addition, due to the clamped structure in which the vibration membrane 3 is fixed to the rigid support post 4 when the microphone is driven, it is not easy to adjust rigidity, making it difficult to additionally enhance sensitivity.
Matters described in the background art section are provided to promote understanding of the background of the present disclosure, which may include a matter that is not a prior art known to those skilled in the art to which the present disclosure pertains.